Lookaround Assertions

Positive lookaround assertion

A positive look around assertion indicates that in order for a particular pattern A to be a match, a pattern B must be coexisting, while pattern B itself is not part of the match. If the matched pattern A comes immediately before the pattern B, it constitutes a positive lookahead, noted as A(?=B); if A comes immediately after B, it constitutes positive lookbehind, noted as (?<=B)A.

eg. 1. Positive lookahead. \\w+(?=ing) matches words that is followed with “ing”, but only considers the part before “ing” as a match. (Recall that \w is equivalent to character class [a-zA-Z0-9_]; also note that a second backslash is needed to escape the first backslash).

library(stringr)x <- c("I sing", "I am singing", "She is dancing")

str_view_all(x, "\\w+(?=ing)")

Output:
[1] │ I <s>ing
[2] │ I am <sing>ing
[3] │ She is <danc>ing

str_extract(x, "\\w+(?=ing)")

Output:
[1] "s"    "sing" "danc"

eg. 2. Positive lookahead. .+(?=@harvard.edu$) matches the username (anything before “@”) of any email address that ends with “@harvard.edu”. Note that the ending anchor $ is inside the positive lookahead parentheses.

emails <- c("john.smith@harvard.edu",            "mary_bosse@harvard.edu",            "Mr.Cool@tesla.com")p <- ".+(?=@harvard.edu$)"

str_view_all(emails, p)

Output:
[1] │ <john.smith>@harvard.edu
[2] │ <mary_bosse>@harvard.edu
[3] │ Mr.Cool@tesla.com

str_extract(emails, p)

Output:
[1] "john.smith" "mary_bosse" NA          

eg. 3. Positive lookbehind. Using the same character vector emails as example, here (?<=@).* matches the domain of an email address, i.e., strings of any length (noted by .*) that follows @ (while the @ sign itself is not part of the match).

str_view_all(emails, "(?<=@).*")

Output:
[1] │ john.smith@<harvard.edu>
[2] │ mary_bosse@<harvard.edu>
[3] │ Mr.Cool@<tesla.com>

str_extract(emails, "(?<=@).*")

Output:
[1] "harvard.edu" "harvard.edu" "tesla.com"  

eg. 4. Positive lookbehind. (?<=\\$\\s{0,1})\\d+ matches a monetary amount, without including the dollar sign and the white space (if any) in the matched pattern. Specifically, the literal dollar sign is noted as \\$, zero or one white space as \\s{0,1}, and one or more digits as \\d+.

a <- c("2 books at $ 33",        "8 pen and 10 pensils at $10")money <- "(?<=\\$\\s{0,1})\\d+"

str_view_all(a, money)

Output:
[1] │ 2 books at $ <33>
[2] │ 8 pen and 10 pensils at $<10>

str_extract(a, money)

Output:
[1] "33" "10"

Negative lookaround assertion

A negative lookaround assertion works very similarly to its positive counterpart, but in the other way around: in order for a particular pattern A to become a match, a particular pattern B must not be coexisting. The negative lookaround includes negative lookahead A(?!B), with matched pattern A immediately ahead of the (undesired) pattern B; and lookbehind (?<!B)A, with pattern A following the unwanted pattern B. We use the exclamation ! the negate sign to create negative assertion, in place of the equation sign = as used in positive assertions.